This invention relates to an excavator having a boom, an arm and a bucket.
In one type of excavator known in the art, a link mechanism in the form of a parallelogram is utilized for maintaining the bucket in a predetermined posture when the bucket is pushed out horizontally.
In this type of excavator, when the earth scooped by the bucket is dumped, the boom is pivotally moved upwardly to a height sufficiently great to dump the earth from the bucket that has been scooped. At this time, the bucket tilts rearwardly as the boom is pivotally moved upwardly, so that there is the danger that the earth may be spilled into the cab. To avoid this danger and also to prepare the excavator for an earth dumping operation, it has hitherto been necessary to perform a complicated operation of pivotally moving the bucket forwardly simultaneously as the boom is pivotally moved upwardly. Various proposals have been made for automatically maintaining the bucket in a predetermined posture when the boom is pivotally moved upwardly to a height sufficiently great to dump the earth, to avoid the problem of the earth being spilled into the cab. One of such proposals is disclosed, for example, in Japanese Patent Application Laid-Open No. 52404/74.
The excavator described in Japanese Patent Application Laid-Open No. 52404/74 includes a master cylinder interposed between the swivelling member and the boom and connected to bucket cylinders via conduits, so that when the boom is pivotally moved upwardly the master cylinder expands to supply hydraulic fluid from the master cylinder to the bucket cylinders to keep the bucket in a predetermined posture.
The following problems have been raised in the excavator of this construction. First, when boom cylinders contract with the bucket in an upper vertical posture position, the master cylinder also contracts and the hydraulic fluid on the bottom side of the master cylinder is introduced into the bucket cylinders via the line, to thereby expand the bucket cylinders. This pivotally moves the bucket rearwardly to a lower vertical posture position. To return the bucket from this position to a horizontal position or an excavation initiating position requires contraction of an arm cylinder. However, a slight contraction of the arm cylinder brings the upper surface of the arm into contact with the back of the bucket. If contraction of the arm cylinder is continued in this condition, the arm cylinder would force the bucket cylinders to contract, so that the time required for this operation would be prolonged and operation efficiency would be reduced. In place of allowing the arm cylinder to force the bucket cylinders to contract, the boom may be pivotally moved downwardly and the bucket may be pivotally moved forwardly. In such case, it is necessary to perform a bucket forward pivoting operation in addition to boom lowering, arm lowering and swivelling operations. This would unnecessarily increase the strain to which the operator is subjected.
Secondly, mounting the master cylinder requires increased strength of the swivelling member and the boom which increases production cost. Also, to connect the master cylinder and the arm cylinder to the lower portion of the boom, it is necessary to mount the master cylinder and the arm cylinder in such a manner that they are displaced sideways to ensure that they do not strike each other. In this construction, a high twisting force would act on the boom due to the thrusts of high magnitude produced by the master cylinder and the arm cylinder. Thus, it would be necessary to increase the strength of the boom by rendering its construction sturdy. This means an increase in the weight of the boom and leads to an increase in production cost. An increase in the weight of the boom makes it imperative to reduce the capacity of the bucket, to avoid an increase in the overall weight of the excavator. Thus the result is a reduction in the performance of the excavator.
Thirdly, when the bucket scoops earth as it is pivotally moved rearwardly while the boom is pivotally moved upwardly, the master cylinder expands as the boom moves upwardly. As a result, part of the hydraulic fluid which should be supplied to the bucket cylinders from a hydraulic pump is introduced into the bottom side of the master cylinder via one of the conduits. Thus, actuation of the bucket cylinders takes more time than is necessary due to its low operation speed, thereby causing a reduction in operation efficiency.
Fourthly, when the bucket is of the tilt-dump type, it is necessary that the bucket be pivotally moved forwardly in its upper vertical posture position to dump earth from the bucket. If the bucket is moved pivotally rearwardly by misoperation, there would be the danger that the earth in the bucket would be dumped into the cab.
The construction of the excavator of the prior art described hereinabove and the problems encountered in the construction and operation of this type of excavator are subsequently to be described by referring to the drawings.